Remove unnecessary update of RegRecord in LSRA (dotnet/coreclr#12474)

Remove unnecessary update of RegRecord in LSRA (dotnet/coreclr#12474)

* Remove unnecessary update of RegRecord

We don't need to clear assignedInterval of regRec,
because it was already cleared by checkAndClearInterval() above.

Signed-off-by: Hyung-Kyu Choi <hk0110.choi@samsung.com>
* Apply review feedback

- Add a assertion and comment for the assertion

Signed-off-by: Hyung-Kyu Choi <hk0110.choi@samsung.com>
Commit migrated from https://github.com/dotnet/coreclr/commit/f0d3c439dc16a3218d53578f14c973696de04cb1